The present invention generally relates to shock absorbing methods and apparatus in which impact forces are absorbed by the compression or other deformation of a spring. More particularly, the present invention concerns methods and apparatus for attenuating vertical oscillations of the body of an aircraft during landing, caused by the compression of the spring components of the landing gear shock struts at and immediately following touchdown.
The principal spring component of the aircraft's landing gear is a compressible body of air confined in a chamber internally of each shock strut. The weight of the aircraft body when supported on these spring components represents a spring mass, and upon landing, the abrupt compression of the spring components produce ensuing, natural oscillations of the body in an up-down (vertical) motion. These vertical oscillations exhibit a frequency and a decaying amplitude determined by the spring and damping characteristics of the shock struts and the weight of the aircraft body. Damping is provided by a hydraulic system internally of the shock strut that forces an incompressible fluid through a flow restrictive orifice in response to strut compression and strut extension, and thereby produces a damping force that opposes any other force tending to compress or extend the shock strut. The forced fluid flow assists the spring in absorbing energy necessary to arrest the downward motion of the aircraft's body. In particular, the spring temporarily stores its share of the absorbed energy by an initial elastic compression (followed by oscillatory reextension, compression, reextension etc.), while the forced fluid flow dissipates its share of energy in the form of heat and a consequent rise in the fluid's temperature. The degree of damping is represented by a damping coefficient, which is defined as a constant that, when multiplied by the square of the velocity of the strut compression or extension, equals the magnitude of the damping force.
Shock struts for commercial jet aircraft are designed with a damping coefficient that is primarily selected to minimize shock or spike loading of the aircraft frame and gear structure when the aircraft touches down at the maximum permissible rate of descent (sinkrate), such as 10 ft/sec. In such case, the largest forces on the landing gears occur during a load stroke cycle of the struts in which the piston and cylinder assembly of each strut is initially compressed (telescoped inwardly) by the touchdown force, and thereafter reextended as the internal spring recoils. For this first cycle, the damping coefficient must not be so great as to rigidify the system and thereby disable the strut from absorbing the energy of the touchdown. Yet the coefficient must be of sufficient magnitude to limit the rate of travel of the piston within the cylinder so that it does not overrun its mechanical limits. These design criteria result in a selected damping coefficient that cannot be varied significantly one way or the other without creating undesirable structural loads on the aircraft and its landing gear during touchdown.
After the first load stroke cycle, vertical oscillations of the sprung mass of the aircraft result in second, third, fourth load stroke cycles, etc. of the shock struts, such oscillations having a decaying amplitude dictated by the damping coefficient that has been selected for the initial touchdown cycle. These oscillations produce periodic, partial-unloading or unweighting of the aircraft wheels (during the rebound portions of the cycles), which in turn, cause variations in the available tire-to-ground frictional engagement, referred to as the ground torque. The level and consistency of the available ground torque affects the braking and ground handling of the aircraft.
Resultant vertical oscillations are accentuated by a trend in aircraft design in which advanced jet transports are provided with a rapid, aerodynamic lift decay substantially immediately after touchdown. The purpose of this post-touchdown lift decay is to place the weight of the aircraft on the ground-engaged wheels, as quickly as practical, to increase the abovementioned ground torque. Post-touchdown lift decay may be accomplished by automatic deployment of wing mounted lift spoilers soon after the wheels have touched ground. In such a case, lift is "spoiled" at a time when the shock struts are being compressed by the touchdown impact. The lost or "spoiler" lift causes the weight of the aircraft body to "plunge" onto to the landing gears, further compressing the struts. Moreover, such "plunging" is somewhat independent of the sinkrate and occurs following sinkrates well below maximum, such as 2-3 ft/sec., which in fact represent the more typical sinkrate.
In some aircraft, lift spoiler produced "plunging" is the prime cause of unwanted post-touchdown oscillations and the term "plunge mode oscillations" has been coined to describe this phenomenon.
Accordingly, it is an object of the present invention to provide an improved damping method and apparatus for a shock absorbing system.
It is another object of the present invention to minimize the amplitude of natural oscillations resulting from the interaction of the spring component and sprung mass of a shock absorbing system, when an abrupt force is applied to the sprung mass through the intermediary of the spring component.
Another object of the present invention is to effectively dampen reciprocatory oscillations occurring in a piston-cylinder shock absorbing assembly of the type having a combined compressible and incompressible fluid system in which the shock absorbing spring is a compressible body of gas (such as air), and the damping is effected by forcing an incompressible fluid (such as oil) through a flow restrictive orifice.
A further object of the present invention is to effectively dampen vertical oscillations of the body of a landing aircraft that follow the touchdown impact and/or the lift spoiler produced "plunge", and that are caused by the natural rebound of the abruptly compressed spring components of the landing gear shock struts. A further and related object is to provide such a method and apparatus while still maintaining the optimum damping coefficient for absorbing touchdown forces at the maximum sinkrate.